Re: Question: Why do you call "AGC" a simple automatic antenna "attenuator" in this forum? #ubitx #ubitxv6

Ashhar Farhan

AGC is a feature and attenuation is a function. Automatic Gain control can work using an electronically controlled attenuation in the signal chain or by using variable gain amplifiers somewhere in the chain.
the golden rule of good signal processing is to control the gain as early in the signal chain as is possible. The best gain control is to put variable resistor right at the antenna terminal. The reason is very simple: the amplifiers down the chain have to handle weaker signals and weaker signals mean better fidelity. An audio AGC in a conventional superhet where most of the gain is in the IF stage will overload even on a moderate signals due to the distortions in IF amplifiers.
Picture this, there is the lowest level signal that the receiver can resolve, this is set by the bandwidth and the noise figure of the receiver. The  highest signal that the receiver can resolve without distorting is determined by the IIP3 and the phase noise of the local oscillator. The range of signals between the minimum discernable signal and the loudest signal that the receiver can handle without noticeable distortion is your dynamic range.
Now, consider what happens if you switch on a 10 db attenuator between the antenna and the receiver. the MDS (minimum discernable signal) has to be 10 times more powerful and on the other hand, the signal level at which the distortion starts to show is also up by 10 db. In effect, to borrrow Rob, NC0B's words, the dynamic range is a moveable window that can be shifted up or down with attenuators.
Most receivers are too sensitive for HF bands. For instance, the ubitx is too sensitive for 40 meters, the atmospheric noise is 100 times more powerful than the internal noise (noise floor) of the receiver. This is wasted dynamic range. if we had thrown in a 20 db attenuator, we would hear exactly the same signals as before but our dynamic range would have been 100 times more. Dynamic range doesn't only apply when you are contesting or having a neighbour who transmits 1000 watts, it is a measure of how sweet the receiver sound is as well.
So, why is ubitx so needlessly sensitive? That is, because it is meant to work from 10 meters to 80 Meters. That sensitivity will be needed when the 15 meter opens up in a few years from now.
RF attenuation does remain the best place to control the gain, even for an AGC system. The challenge is to build a smoothly varying attenuator that doesn't need any active devices. Active devices in wide open receivers like the ubitx can severally compromise the receiver performance. 
I am planning to do a video to explain who careful gain distribution is the key to a good radio design.
- f

On Fri, Nov 20, 2020 at 9:38 AM TD <dlee@...> wrote:
> "what component is P1" ?????

I figured it was a pot but never seen that symbol used before.

Darrell Lee
Advanced Data Systems, Inc.
2801 Wade Hampton Blvd.
Suite 115-153
Taylors, SC 29687
864-230-9626 | dlee@...

On 11/19/2020 01:15 PM, IW4AJR Loris wrote:
> Hello TD ...
> "what component is P1" ?????
> maybe it is simply a resistive trimmer or a potentiometer ???? perhaps
> don't tell me you don't know what P1 is because I don't believe it!
> this diagram is just an example of how a simple dynamic compressor
> with a handful of passive components can be made, how to insert it and
> where to insert it in the µBITX is something you have to decide,
> study it a bit and look at the diagram of the part receiver of the
> µBITX (from the volume potentiometer to the speaker) and see if you
> need it as an input to the BF amplifier (LM386) or if you want to
> insert it in parallel with the output before the headphone jack.
> hi TD ... good job! greetings from IW4AJR Loris
> Links:
> ------
> [1]
> [2]
> [3]
> [4]
> [5]
> [6]
> [7]

Join to automatically receive all group messages.